This application is related to Japanese Patent Application No. Hei 11(1999)-039578 filed on Feb. 18, 1999, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a plasma generation apparatus and method for manufacture of a functional thin film of a semiconductor or the like by plasma-enhanced chemical vapor deposition. In particular, the invention relates to apparatuses for performing plasma-enhanced chemical evaporation of or plasma etching on a semiconductor thin film such as a hydrogenated amorphous silicon thin film or an insulating film with high-frequency plasma enhancement.
2. Description of the Related Art
The plasma-enhanced chemical vapor deposition (CVD) that forms a semiconductor film by using plasma is widely employed in manufacturing electronic devices such as integrated circuits, liquid crystal displays, and solar panels. A general method of plasma-enhanced CVD will be described with reference to FIG. 8.
Electrically insulated conductive plates 102 and 103 are provided in a vacuum container 101 as electrodes that are opposed to each other. High-frequency power having a frequency of 13.56 MHz, for example, is supplied from a plasma generation power source 104 via an adjustment circuit 105. When a high-frequency voltage is applied to the electrode 102, glow discharge occurs between the electrode 102 and the ground-potential electrode 103, whereby plasma is generated between the electrodes 102 and 103. A material gas G that is caused to flow between the electrodes 102 and 103 is dissociated. With the presence of the dissociated material gas G, a semiconductor film is formed on a substrate S that is mounted on the electrode 103.
In recent years, to increase the film forming rate by increasing the plasma density and to form a high-quality film by reducing the degree of ion damage of films by decreasing the ion sheath voltage, apparatuses have been developed that use a power source of higher frequency bands, in particular, an RF band (13.56 MHz), a VHF band (tens of megahertz), and a UHF band (hundreds of megahertz). In particular, it has already been reported that satisfactory film formation can be attained by a method in which plasma CVD is performed by using small-area electrodes and applying high-frequency power of a VHF band.
Further, in recent years, functional thin films measuring about 1 meter have come to be desired for electronic devices such as liquid crystal displays and amorphous solar panels and attempts to develop a large-area film forming technique using the above manufacturing method have been made. One of the important subjects in attaining the large-area film formation is to develop a technique for forming a film that is uniform in thickness and film quality. An example of such a technique is such that the surface shapes of electrodes are modified so that the electric field strength between the electrodes becomes uniform in the same plane. One factor that relates to a phenomenon that the uniformity of the film thickness and (or) the film quality becomes lower as the film area is increased is a power feeding method for the glow discharge electrode.
In the conventional feeding method for the glow discharge electrode, the skin effect causes supplied high-frequency waves to travel through the surface of the outer wall of a metal reaction container and hence the high-frequency waves are applied to the cathode electrode, for example, from a peripheral portion of the outer wall of the reaction container. This feeding method has problems that the feeding position with respect to the electrode cannot be changed even if the feeding point (power source connecting portion) is changed and that the impedance to the electrode as viewed from the feeding point is large.
A method in which a plurality of contact bodies are interposed between the power source connecting portion and the electrode surface (Japanese Unexamined Patent Publication No. Sho 64(1989)-89316) is known as a method for decreasing the impedance from the power source to the electrode.
Although plasma CVD apparatuses using high-frequency power of a VHF band or a UHF band provide the advantage of improved film quality as mentioned above, they have a problem that the film forming rate and the film quality become non-uniform when those apparatuses are applied to formation of a large-area film. For example, if high-frequency power of VHF or the like is used in a large-area film forming apparatus, the film forming rate at an electrode peripheral portion is much smaller than at an electrode central portion.
Non-uniformity in film forming rate in the electrode plane and resulting non-uniformity in film quality lead to reduction in the characteristics of an amorphous silicon solar panel, for example, and hence they are serious problems in film forming apparatuses for solar panels. It is generally known that the non-uniformity in film forming rate is due to a non-uniform in-plane profile of the electric field strength between the electrodes and that the electric field strength profile in the electrode plane becomes less uniform as the wavelength of high-frequency waves applied approaches the electrode length. As the electric field strength profile becomes less uniform, a variation occurs in the plasma density profile and the radical density profile and hence in the film forming rate profile. In plasma etching apparatuses, a variation similarly occurs in the profile of the electric field strength between the electrodes, which causes a problem that the etching rate varies.
On the other hand, the method of the above-mentioned publication No. Sho 64(1989)-89316 has been proposed to make the in-plane profile of the high-frequency electric field strength uniform. However, this method cannot decrease the impedance to a large extent because of the skin effect. In particular, when high-frequency power of a VHF band or a UHF band is used, the impedance becomes large and the profile of the electric field strength between the electrodes becomes non-uniform.
The present invention has been made to solve the above problems in the art, and an object of the invention is therefore to provide a plasma generation apparatus and method capable of making the film forming rate and the film quality uniform by preventing the impedance from becoming unduly large and thereby making the profile of the high-frequency electric field strength in the electrode plane uniform even in a case where in a semiconductor film forming apparatus or the like the frequency of power supplied to an electrode is increased to improve the film quality to such an extent that the electrode length is no longer negligible with respect to the wavelength of high-frequency waves.
According to the present invention, there is provided a plasma generation apparatus comprising: a high-frequency power source section; a reaction container to which a material gas is to be supplied; a pair of electrodes that are provided in the reaction container so as to be opposed to each other to generate plasma in between when supplied with high-frequency power by the high-frequency power source section; a conductive connection member that is part of an outer wall of the reaction container and that has, on one side, at least one power source connecting point that is connected to the high-frequency power source section and has, on the other side, at least one electrode connecting point that is connected to the one of the pair of electrodes; and high-frequency current transmitting means provided in the conductive connection member in the vicinity of the at least one power source connecting point, for adjusting impedance between the at least one power source connecting point and the at least one electrode connecting point.
That is, the high-frequency power source section is connected to one of the electrodes (e.g., a cathode electrode) via the conductive connection member that is part of the outer wall of the reaction container, and the high-frequency current transmitting means for adjusting the impedance is provided in the conductive connection member. Therefore, high-frequency current is transmitted from the power source connecting points to the electrode connecting points via the high-frequency current transmitting means. This prevents increase of the high-frequency current propagation path length and concentration of propagation paths that are associated with the skin effect, that is, a phenomenon that as the frequency increases high-frequency current propagates the surface and its vicinity of a conductor (in this case, the conductive connection member) more selectively.
According to a specific embodiment of the invention for improving the high-frequency current propagation paths, the conductive connection member is a conductive plate having the power source connecting points on the outside surface and the electrode connecting points on the inside surface, and the high-frequency current transmitting means is through-holes that penetrate the conductive plate in the inside-outside direction. With this structure, high-frequency current that is supplied to the power source connecting points formed in the vicinity of the respective through-holes are transmitted to the electrode connecting points in such a manner as to selectively propagate through the surfaces of the through-holes and their vicinities. Therefore, the impedance of the conductive connection member can be reduced.
Therefore, the through-holes of the conductive connection member can prevent the electric field strength from decreasing at portions on the periphery of the electrode connecting points that are distant from the power source connecting points and the uniformity of the electric field strength can be made higher. As a result, when a film is formed by placing a subject substrate between the electrodes, the film forming rate profile and the film quality profile in the electrode plane formed by the pair of electrodes can be given sufficient levels of uniformity.
Further, the frequency of high-frequency power supplied can be increased. Even where the electrode length is relatively so long as not to be negligible with respect to the wavelength of a high-frequency electric field, a sufficient level of uniformity of the electric field strength is secured. High-frequency electromagnetic waves of a VHF band or a UHF band can be used for plasma CVD. This makes it possible to utilize the advantages of plasma CVD and plasma etching that use high-frequency electromagnetic waves, such as an increased film forming rate by virtue of an increased plasma density and reduced ion damage by virtue of a decreased ion sheath voltage.
In the invention, it is preferable that not only the power source connecting points but also the electrode connecting points be provided in the vicinity of the high-frequency current transmitting means. There may be provided only one set of a power source connecting point and an electrode connecting point.
However, from the viewpoint of the uniformity of the electric field strength, it is preferable that there be provided plural sets of a power source connecting point and an electrode connecting point.
By inserting an insulative plug in each through-hole, the reaction container can be kept airtight.
Each of the through-holes according to the invention may assume a circle, a polygon, a rectangle, or a combination of any of those.
By forming the through-holes in such a manner that the diameter of an area-equivalent circle is larger than 1 mm and smaller than the interval between adjacent ones of the through-holes, the plasma generation apparatus can be used for plasma CVD using high-frequency waves of a VHF band or a UHF band. The diameter of an area-equivalent circle, which is the diameter of a circle having the same area as the area of a through-hole, is used here to define the size of a through-hole, because through-holes having various shapes are covered by the invention.
It is preferable that the interval between the through-holes be shorter than a half of the wavelength of high-frequency power supplied. This makes it possible to improve the film quality by increasing the frequency of high-frequency power. Even where the length of the pair of electrodes is so long as not to be negligible with respect to the wavelength, the number of power source connecting points can be increased by shortening the interval between the through-holes, whereby the in-plane profile of the high-frequency electric field strength can be made uniform and the film forming rate and the film quality can thereby be made uniform in the electrode plane.
By providing each through-hole with a gas supply inlet through which a material gas is supplied to the reaction container, a material gas can be supplied to the space between the electrodes at a higher level of uniformity. A specific example is such that a gas supply inlet penetrates an insulative plug that is inserted in each through-hole. By supplying a larger amount of material gas through through-holes corresponding to electrode portions where the film forming rate would otherwise be low, the degree of dissociation of the material gas by plasma at those portions can be increased, whereby the film forming rate can be increased there and its in-plane uniformity can be made higher. This is particularly effective in a plasma-enhanced chemical evaporation apparatus that performs chemical evaporation on a substrate by using a material gas supplied to the reaction container.
By providing the high-frequency power source section with an adjustment circuit that supplies different amounts of power to the respective power source connecting points, the problem of reduction of the film forming rate at an electrode peripheral portion can be solved. Specifically, by inputting, via the conductive connection member, larger power to electrode connecting points corresponding to electrode portions where the film forming rate would otherwise be low than to other electrode connecting points, the uniformity of the electric field strength can be made higher and hence the film forming rate and the film quality can be given a sufficient level of uniformity. A specific example of the adjustment circuit is a circuit in which a variable capacitor is provided between the power source and the ground and a series connection of a coil and a variable capacitor is provided between the power source and each electrode connecting point.
By forming the high-frequency power source section so that it supplies high-frequency waves having a frequency of 20-200 MHz to the one of the electrodes, the film forming rate can be increased by virtue of an increased plasma density and ion damage to the formed film can be reduced by virtue of a decreased ion sheath voltage. These make it possible to form what is called a high-quality a xe2x80x94Si:H film that exhibits a large ratio of the light conductivity to the dark conductivity and has a small density of defects in the film. This is particularly effective in a plasma-enhanced chemical evaporation apparatus that is used to form semiconductor thin films for solar panels.
The plasma generation apparatus according to the invention can be constructed as a plasma-enhanced chemical evaporation apparatus that performs chemical evaporation on a substrate by using a material gas supplied to the reaction container or a plasma etching apparatus that performs etching on a substrate.
In this case, as described above, the film forming rate by chemical evaporation or the etching rate can be increased and its in-plane uniformity can be made higher, whereby the advantages of plasma CVD or plasma etching can fully be utilized.
According to the present invention, there is provided a plasma generation method comprising the steps of: preparing a high-frequency power source section, a reaction container to which a material gas is to be supplied, a pair of electrodes that are provided in the reaction container so as to be opposed to each other to generate plasma in between when supplied with high-frequency power by the high-frequency power source section, and a conductive connection member that is part of an outer wall of the reaction container and that has, on one side, at least one power source connecting point that is connected to the high-frequency power source section and has, on the other side, at least one electrode connecting point that is connected to the one of the pair of electrodes; and supplying the high-frequency power from the high-frequency power source section to the pair of electrodes via high-frequency current transmitting means provided in the conductive connection member in the vicinity of the at least one power source connecting point, for adjusting impedance between the at least one power source connecting point and the at least one electrode connecting point.